1. Field of the Invention
The present invention relates to a superconducting quantum interference device (called xe2x80x9cSQUIDxe2x80x9d in the specification), and more specifically to a novel SQUID having a superconducting current path constituted of an oxide superconductor material.
2. Description of Related Art
Of electronic devices utilizing the superconduction phenomenon, a SQUID is one of the devices most advanced in reduction into practice.
The SQUID is formed of a closed-loop superconducting current path having one or two weak links inserted thereto. A superconducting current flowing through the closed-loop superconducting current path cannot exceed a critical current of the weak link portion, and therefore, a current density in the closed-loop superconducting current path is extremely small. Accordingly, the momentum of cooper pairs existing in the superconducting current path is small, and on the other hand, the wavelength of a corresponding electron wave is extremely long. As a result, it can be regarded that the phase is the same in different portions within the superconducting current path. However, if a magnetic field is applied to this superconducting current path, a phase difference occurs in the superconducting current path. By utilizing this unique phenomenon, the SQUID can be used as a magnetic sensor having an extremely high sensitivity.
In fact, the SQUID has already actually been used not only in a fundamental measurement field for a magnetization meter, a NMR, a magnetic thermometer, etc. but also in a medical field for a magnetic cardiograph, an electroencephalograph, a magnetic tracer, etc., and in the field of earth science for a geomagnetism observation, an earthquake prediction, a resource prospecting, etc.
On the other hand, the superconducting material known in the prior art had become a superconductor only at an extremely low temperature not greater than a liquid helium temperature, and therefore, it had not been considered to practically utilize the superconducting material. However, since it was found in 1986 that compound oxide sintered material such as (La, Ba)2CuO4 or (La, Sr)2CuO4 are a superconductor material having a high critical temperature (Tc), it has been confirmed from one to another than a compound oxide such as a Yxe2x80x94Baxe2x80x94Cuxe2x80x94O type compound oxide or a Bixe2x80x94Caxe2x80x94Srxe2x80x94Cuxe2x80x94O type compound oxide shows a superconduction characteristics at an extremely high temperature. This material showing the superconduction characteristics at the high temperature permits to use an inexpensive liquid nitrogen for a cooling medium, application of superconduction technique has abruptly been put under study as an actual matter.
Therefore, if the oxide superconductor is used in the SQUID, it is expected to further promote spread of the SQUID which has been actually used in the above mentioned various fields. However, the SQUID actually formed of the oxide superconductor has internal noise generated by the SQUID itself, and therefore, can have only a substantially low sensitivity. When the SQUID is used as a sensor, it is possible to eliminate external noises by using a high degree of gradiometer, but it is very difficult to eliminate internal noises. Because of this, it is difficult to use the SQUID composed of an oxide superconductor, as a low-noise high-sensitivity sensor.
Accordingly, it is an object of the present invention to provide a novel SQUID composed of an oxide superconductor material, which has overcome the above mentioned defect of the conventional one and which has a low noise.
According to the present invention, there is provided a SQUID including a substrate and a superconducting current path of a patterned oxide superconductor material thin film formed on a surface of the substrate, a c-axis of an oxide crystal of the oxide superconductor material thin film being oriented in parallel to the surface of the substrate.
As seen from the above, the SQUID in accordance with the present invention is characterized in that the c-axis of the oxide crystal of the oxide superconductor material thin film forming the superconducting current path is oriented in parallel to the surface of the substrate.
It has been known that, general oxide superconduction materials such as a high-Tc copper-oxide type oxide superconductor material typified by a Yxe2x80x94Baxe2x80x94Cuxe2x80x94O type compound oxide superconductor material, a Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O type compound oxide superconductor material, and a TIxe2x80x94Baxe2x80x94Caxe2x80x94Cuxe2x80x94O type compound oxide superconductor material, have remarkable anisotropy in its crystal structure, concerning characteristics including a critical current density and others. For example, the typical Yxe2x80x94Baxe2x80x94Cuxe2x80x94O type compound oxide superconductor material permits a larger superconducting current to flow in a direction perpendicular to a c-axis of the crystal than in a direction of the c-axis of the crystal. Therefore, when the SQUID is formed of the oxide superconductor thin film, since the superconducting current of the SQUID flows in parallel to the substrate, it has been an ordinary practice to utilize an oxide superconductor thin film having its c-axis oriented perpendicularly to the substrate.
However, examining in detail the operation of the above mentioned conventional SQUID, it has been found that in case of the oxide superconductor, a magnetic flux creep is large within a plane perpendicular to the c-axis of the crystal, and therefore, the noise of the SQUID rather becomes large. On the other hand, considering the function of the SQUID, it is not necessary to cause a large current to flow. As a result, it has been concluded that, rather, it is necessary to form the superconducting current path by giving importance to a magnitude of a pinning effect. The present invention has been completed on the basis of this recognition.
Namely, in the above mentioned SQUID in accordance with the present invention, the c-axis of the oxide crystal of the oxide superconductor material thin film forming the superconducting current path is oriented in parallel to the surface of the substrate. The superconducting current path constituted of such an oxide superconductor material thin film is even low in the critical current density, but the flux creep is small. Therefore, it can realize a low noise SQUID.
The present invention can be applied to a SQUID formed of any oxide superconductor material thin film having anisotropy. For example, the present invention can be applied not only to the SQUID formed of a Y type compound oxide superconductor thin film, but also to SQUIDs formed of other copper-oxide type compound oxide superconductor thin film having anisotropy of a pinning effect, including a so-called Bi type compound oxide superconductor thin film, and a so-called Tl type compound oxide superconductor thin film.
A preferred substrate on which the above mentioned SQUID is formed, includes a MgO single crystal, a SrTiO3 single crystal, a LaAlO3 single crystal, a LaGaO3 single crystal, a Al2O3 single crystal, and a ZrO2 single crystal.
For example, the oxide superconductor thin film having the c-axis in parallel to the substrate can be deposited by using, for example, a (100) surface of a MgO single crystal substrate, a (110) surface of a SrTiO3 single crystal substrate and a (001) surface of a CdNdAlO4 single crystal substrate, as a deposition surface on which the oxide superconductor thin film is deposited.
In addition, the oxide superconductor thin film having the c-axis in parallel to the substrate can be preferably deposited by maintaining a substrate at a temperature which is lower than a substrate temperature which enables a deposited layer to have a c-axis perpendicular to the substrate, by a temperature difference within a range of 10xc2x0 C. to 100xc2x0 C., and more preferably, a range of a few tens xc2x0 C. to 100xc2x0 C.
In one embodiment, the weak link of SQUID is formed of a portion of the oxide superconductor material thin film positioned just on a step portion of the substrate. The height of this step can be freely selected from the range of 800xc3x85 to 3,000xc3x85. In order to ensure that the weak link is formed of the portion of the oxide superconductor material thin film positioned just on the step portion of the substrate, the thickness of the oxide superconductor material thin film has to be properly selected. If the thickness of the oxide superconductor material thin film is too thin in comparison with the height of the step of the substrate, the oxide superconductor material thin film would be broken at the step portion of the substrate. On the other hand, if the thickness of the oxide superconductor material thin film is too thick in comparison with the height of the step of the substrate, the weak link could not be formed of the portion of the oxide superconductor material thin film positioned just on the step of the substrate. In the case of the step portion having the height of 1,000xc3x85, the thickness of the oxide superconductor material thin film forming the superconducting current path is preferably on the order of 500xc3x85 to 5,000xc3x85. In addition, the oxide superconductor material thin film forming the superconducting current path can be formed by a physical deposition or a chemical deposition, both of which are well known to persons skilled in the art. In particular, a sputtering or an ion plating can be advantageously utilized.
The above and other objects, features and advantages of the present invention will be apparent from the following description of a preferred embodiment of the invention with reference to the accompanying drawings. However, it should be noted that the following disclosure is merely one embodiment for making it easier to understand the present invention, and the present invention is in no way limited to the details of the illustrated structures.